The invention relates to a method for manufacturing a mixture of sensing material and more particularly to a method for making a mixture of sensing material having improved ion efficient characteristics for a given volume of material heated to a predetermined temperature to provide enhanced indicating instrument sensitivity when the sensing mixture is assembled in an electrical gas detection device and exposed to a flow of halogen vapor.
It is well known to use electrical gas detection devices to detect ions collected at oppositely charged emitter and collector electrodes near which a mixture of ion emitting material is positioned. Typically, such a sensing device includes a cylindrical negative electrode having a helically wound positive heater electrode coaxially disposed around it. The negative electrode is formed of a relatively inert material such as platinum and is filled with a mixture of sensing material such as granules of ceramic coated with an alkali metal. In using a device of this type to detect gas leaks; for example, leaks in refrigeration systems or other systems of piping in which a halogen tracer gas has been introduced, it is only necessary to draw a sample of gas from adjacent the system, through a suitable tubular probe, into contact with the sensor element of the indicating device. As the sample gas is passed through or near the heated electrodes, sensing material located within the negative electrode induces a flow of ions from the alkali source mixture at a rate that varies dependent on the concentration of halogen gas in the sample gas.
Examples of such prior art vapor detectors are disclosed in U.S. Pat. No. 2,795,716-Roberts and U.S. Pat. No. 3,439,262-Roberts, both of which are assigned to the assignee of the present invention. As explained in yet another patent, U.S. Pat. No. 3,471,746-Roberts, also assigned to the same assignee, one operating difficulty encountered with the use of electrical discharge vapor detecting devices is that they are highly sensitive to variations in temperature of the mixture of sensing material. Thus, it is desirable to either provide a constant heat input to the sensing material, based upon empirical heat loss data for a given sensing element, or to provide an accurate temperature control for the sensor in order to assure accurate and reliable indications of current rate between the emitter and collector of the instrument. The need to control the heater temperature of such an instrument is complicated by the fact that the heat developed by the heating coil varies as the square of the voltage applied to it and the sensitivity of the sensing mixture increases logarithmically with respect to temperature variations.
Practical engineering considerations such as limitations on size and economic constraints on design costs make it particularly difficult to provide the desired level of accuracy in vapor detecting instruments when the instruments must be made small enough to be conveniently portable. On the other hand, the large number of leak detecting applications in which it is desirable to have a portable gas detecting instrument make it very desirable to overcome these difficulties. Thus, it would be particularly useful to provide a halogen detecting instrument that incorporates an alkali metal sensing mixture in which a relatively small volume of sensing material affords a sufficiently large and uniform current flow to provide an accurate indication of a detected halogen vapor when the mixture of sensing material is heated to a given temperature. Moreover, it would be advantageous to provide a method of making such a sensing mixture that enables the mixture to be heated to its most ion efficient state by a heater that requires a relatively low level of energizing power.
Accordingly, it is an object of the present invention to provide a method for making a sensing mixture for a halogen gas sensor instrument that is capable of producing a relatively large indicating current from a small volume of sensing material that can be readily heated to an optimum emission temperature by a small heater current.
Another object of the invention is to provide a method of making a mixture of sensing material for a halogen gas detector in which an unusually large area of alkali metal is dispersed throughout the interstices of a small volume of supporting granular material.
Yet another object of the invention is to provide a method of manufacturing a mixture of sensing material for a halogen gas sensor such that the heat sensing mixture is economically produced and affords reliable and efficient ion emission characteristics.
Additional objects and advantages of the invention will be apparent to those skilled in the art from the description of it that follows considered in conjunction with the accompanying drawing.